1. Field
Embodiments described herein relate generally to a Ni-base alloy for forging or rolling and a steam turbine component made of the same and particularly to a Ni-base alloy for forging or rolling and a steam turbine component made of the same that can maintain productivity such as hot workability and weldability as well as improve high-temperature strength.
2. Description of the Related Art
In a thermal power plant including a steam turbine, an art to reduce carbon dioxide emission has been drawing attention in view of global environmental protection, and a need for highly efficient power generation has been increasing.
For higher efficiency of the power generation of a steam turbine, it is effective to increase the temperature of turbine steam. Recently, a thermal power plant including a steam turbine uses the steam whose temperature is equal to or higher than 600° C. The future trend is toward a higher steam temperature up to 650° C., further 700° C., or over 700° C.
High-temperature pipes, flanges, elbows, turbine casings, valve casings and nozzle boxes of the steam turbine into which high-temperature, high-pressure steam flows as a working fluid can be regarded as a kind of a high-temperature pressure vessel receiving a high inner pressure under a high-temperature environment. Therefore, the above components are required to endure high temperature and high stress, and materials for the above components are required to be excellent in strength, ductility and toughness in a high-temperature range. The materials also need to have excellent steam oxidation resistance because of long use at high temperature for a long time.
In the above components, the high-temperature pipe and the flange are almost formed by hot working such as forging, extrusion and drawing. Meanwhile, the elbow, the turbine casing, the valve casing and the nozzle boxes are in many cases formed by casting. However, in order to avoid a reduction in the quality of the components associated with the failure of casting that occurs in the process of casting the components, high-quality cast products may be made by producing an ingot after melting and refining and forging the ingot into the shape of the components. Therefore, the materials applied need to be excellent in hot workability.
The above components constitute part of the turbine and are structurally used in combination with other components. For example, the following components are fitted into the turbine casing: turbine rotors that are rotated by steam, rotor blades, nozzles (stator blades), tie bolts, nozzle boxes and the like. Structural designing is easy when the thermal expansion coefficient of the turbine casing is substantially at the same level as the thermal expansion coefficient of the inner structure components, which also leads to a significant improvement in reliability for long-term operation. Given the fact that the locally-generated thermal stress decreases for large structures as the thermal expansion coefficient is lowered, structural designing becomes easier and long-term reliability improves.
Instead of turning one component into an integrated forging product, the shape of the component may be formed by welding forging segments together. In this case, the segments made of the same material or the segments made of different materials having different chemical compositions may be welded together. Therefore, the materials also need to be excellent in weldability.
At present, typical materials as a Ni-base alloys whose use in the application where the steam temperature is 700° C. or over 700 00° C. is under consideration to the above components are an Inconel 617 alloy (IN617, manufactured by Special Metals Corporation), an Inconel 625 alloy (IN625, manufactured by Special Metals Corporation), an Inconel 740 alloy (IN740, manufactured by Special Metals Corporation), and HR6W (manufactured by Sumitomo Metal Industries, Ltd.)
IN617, IN625 and HR6W are excellent in creep rupture elongation, steam oxidation resistance, hot workability and weldability. However, the creep rupture strengths of IN617, IN625 and HR6W are not sufficient and the thermal expansion coefficients thereof are relatively large. Therefore, the high-temperature components to which the above materials are applied entail difficulty in designing the structures and there are many problems for long-term, stable operation at high temperature. IN740 is excellent in creep rupture strength, steam oxidation resistance and weldability. However, the creep rupture elongation of IN740 is low and the thermal expansion coefficient thereof is relatively large. Therefore, the high-temperature components to which the above material is applied entail difficulty in designing the structures and there are many problems for long-term, stable operation at high temperature.
Moreover, typical materials as a Ni-base alloys whose use in the application of the rotor blades, stator blades and tie bolts at a steam temperature of 700° C. or over 700° C. is under consideration to the above components are an Inconel 713C alloy (IN713C), an Udimet 520 alloy (U520), an Inconel X-750 alloy (X-750), a M252 alloy and an Inconel 718 alloy (IN718). IN713C and U520 are excellent in creep rupture strength. However, the creep rupture elongation of IN713C and U520 is small; IN713C and U520 are not good in hot workability. Even though the thermal expansion coefficient of IN713C is relatively low, IN713C is not good in steam oxidation resistance. Meanwhile, U520 is excellent in steam oxidation resistance. However, the thermal expansion coefficient of U520 is relatively high. X-750 is excellent in creep rupture strength and creep rupture elongation but not good in hot workability and steam oxidation resistance; the thermal expansion coefficient of X-750 is relatively high. M252 is excellent in creep rupture strength, creep rupture elongation and steam oxidation resistance and has a relatively low thermal expansion coefficient. However, M252 is not good in hot workability. IN718 is excellent in creep rupture elongation, hot workability and steam oxidation resistance but not good in creep rupture strength; the thermal expansion coefficient of IN718 is relatively high.